Spark plugs for use in internal combustion engines such as automobile engines each have a spark plug insulator (also referred to simply as “insulator”) formed from, for example, an alumina-based sintered material containing alumina (Al2O3) as a principal component. This insulator is formed from such an alumina-based sintered material because the alumina-based sintered material is excellent in heat resistance, mechanical strength, and the like. In order to obtain such an alumina-based sintered material, for example, a three-component sintering aid composed of, for example, silicon oxide (SiO2), calcium monoxide (CaO), and magnesium monoxide (MgO) has been used for the purpose of lowering the firing temperature and improving sinterability.
The temperature in a combustion chamber of an internal combustion engine to which such a spark plug is attached sometimes reaches about 700° C., for example. Therefore, the spark plug is required to exert excellent withstand voltage performance in a temperature range from the room temperature to about 700° C. Alumina-based sintered materials have been proposed which are suitably used for insulators or the like of spark plugs exerting the withstand voltage performance.
For example, Japanese Patent Application Laid-Open (kokai) No. 2001-155546 discloses “ . . . an insulator for a spark plug, which comprises an alumina-based sintered body comprising: Al2O3 (alumina) as a main component; and at least one component (hereinafter referred to as “E. component”) selected from the group consisting of Ca (calcium) component, Sr (strontium) component and Ba (barium) component, wherein at least part of the alumina-based sintered body comprises particles comprising a compound comprising the E. component and Al (aluminum) component, the compound having a molar ratio of the Al component to the E. component of 4.5 to 6.7 as calculated in terms of oxides thereof, and has a relative density of 90% or more.” (see claim 1 of Japanese Patent Application Laid-Open (kokai) No. 2001-155546). Japanese Patent Application Laid-Open (kokai) No. 2001-155546 indicates that this technique can provide a spark plug having an insulator which is less liable to occurrence of dielectric breakdown due to the effect of residual pores or low-melting glass phases present on boundaries of the alumina-based sintered body, and exhibits a higher dielectric strength at a temperature as high as around 700° C. than the conventional materials (see, for example, paragraph [0007] of Japanese Patent Application Laid-Open (kokai) No. 2001-155546).
Meanwhile, PCT International Publication No. WO 2009/119098, for the purpose of providing a spark plug having an insulator that exerts high withstand voltage characteristics and high-temperature strength (see paragraph [0014] of PCT International Publication No. WO 2009/119098), discloses “A spark plug . . . the insulator is formed from a dense alumina-based sintered material having a mean crystal grain size DA (Al) of 1.50 μm or more; the alumina-based sintered material contains an Si component and, among configured to transmit torque on substantially a 1:1 basis between its proximal and group 2 elements (the Group included in the periodic table defined by Recommendations 1990, IUPAC), Mg and Ba, as essential components, and a group 2 element (2A) component containing at least one element other than Mg and Ba, and a rare earth element (RE) component, wherein the ratio of the Si component content S (oxide-reduced mass %) to the sum (S+A) of S and the group 2 element (2A) component content A (oxide-reduced mass %) is 0.60 or higher” (see claim 1 of PCT International Publication No. WO 2009/119098).
Japanese Patent Application Laid-Open (kokai) 2014-187004, for the purpose of improving the strength and the withstand voltage performance, discloses “an insulator . . . wherein a ratio between a content of a rare earth element as reduced to oxide and expressed in percent by mass and a content of a group 2 element (included in the periodic table defined by Recommendations 1990, IUPAC) as reduced to oxide and expressed in percent by mass, satisfies 0.1≦content of rare earth element/content of group 2 element≦1.4, and a ratio between the content of the rare earth element and a content of barium oxide as reduced to oxide and expressed in percent by mass, satisfies 0.2≦content of barium oxide/content of rare earth element≦0.8, wherein at least one virtual rectangular frame of 7.5 μm×50 μm that encloses a crystal containing the rare earth element is present in an arbitrary region of 630 μm×480 μm at a cross section of the sintered body, and an occupation ratio of an area of the crystal containing the rare earth element to an area of the rectangular frame is 5% or more, and when the rectangular frame is divided into three division regions in a direction of a long side thereof, among occupation ratios of areas of the crystal containing the rare earth element in the respective division regions, a ratio between the occupation ratio of the maximum area and the occupation ratio of the minimum area is 5.5 or less” (see claim 1 of Japanese Patent Application Laid-Open (kokai) 2014-187004).